Summary

On 23 April 2011, at 1644 Eastern Standard Time, a
loss of separation assurance (LOSA) occurred between a Boeing
B737-8BK (737), registered VH-VOB, and an Airbus Industrie A320-232
(A320), registered VH-VGZ, near Armidale Airport, New South Wales
(NSW).

Both aircraft were conducting scheduled passenger
services at flight level 320 on the same one way air route, H62,
until position Mount Sandon, where the leading 737 aircraft would
continue tracking south for Sydney, while the following A320
aircraft was flight planned to track left to Williamtown.

As the 737 was required to enter a holding pattern
for sequencing into Sydney, the air traffic controller approved a
speed reduction for the aircraft, but did not identify that the
longitudinal separation between it and the following A320, was
closing.

When the 737 flight crew provided the controller with
their aircraft's speed information for the benefit of the following
traffic, the controller identified the potential confliction and
issued the A320 with a vectoring instruction to maintain
separation. Although the separation between the two aircraft did
not reduce below the required standard, a LOSA had occurred due to
the closing longitudinal proximity between the two aircraft.

Airservices Australia (Airservices) advised that a
review of LOSA and breakdown of separation occurrences would be
undertaken to determine whether speed differential and aircraft
performance were significant contributing factors and if additional
refresher training was required for controllers.

This occurrence highlights the need for awareness of
the effects of low workload on performance, identification of
lowered vigilance and subsequent action or implementation of
strategies to maintain safe operations. It also demonstrates how
appropriate flight crew communication enhanced the situational
awareness of the controller. In addition, the knowledge and
application of effective compromised separation recovery techniques
by the controller was integral in managing the situation.